Transparent conductive films for use as the window layer of a CIGS (copper indium gallium diselenide) thin-film solar cell should have n-type conductivity. The performance characteristics which these transparent conductive films are required to have include the following:
(1) a low resistivity, because of the use thereof as an upper electrode (the sheet resistance thereof is theoretically as low as possible, desirably 10 .OMEGA./.quadrature. or lower); PA0 (2) a high transmittance, because of the function thereof as a window layer through which incident light should pass (the transmittance thereof is desirably 90% or higher (at 800 nm)); and PA0 (3) avoiding impairment of junction characteristics, because the CIGS thin-film solar cell has a stacked structure. PA0 (1) It is possible to form a transparent conductive film having any desired resistivity by regulating the amount of a dopant at low substrate temperature which mutual diffusion of each constituent of a CIGS thin-film solar cell is hard to occur. PA0 (2) Since MOCVD is a technique of depositing a transparent conductive film based on a chemical reaction, a hard zinc oxide (ZnO) film can be deposited under mild conditions without damaging the soft light absorbing layer of a CIGS solar cell. PA0 (3) By regulating its surface roughness, the transparent conductive film can be made to have a surface texture which produces a light trapping effect, and thus is expected to partly function as an antireflection film. PA0 (1) It is impossible to independently control the transmittance and the resistivity of a transparent conductive film (the optimal values thereof are a compromise between the two properties). PA0 (2) It is necessary to heat the substrate to accelerate the chemical reaction, and the resulting thermal stress causes the substrate to suffer elastic deformation, which in turn is apt to cause troubles including film peeling and generation of defects. PA0 (3) A layer having high resistance deposits in the initial stage of the formation of a transparent conductive film of zinc oxide (ZnO), so that the film finally obtained cannot have low resistance throughout the whole thickness. PA0 (4) Since the deposition of a transparent conductive film is based on a chemical reaction, the film production requires a significant amount of time (for instance, the deposition of a ZnO film with a thickness of 2 .mu.m requires about 1 hour depending on the size of apparatus for this purpose). PA0 (5) Since the production equipment includes a batch reactor, the method has poor production efficiency in industrial production. PA0 (6) In producing a large-area thin-film solar cell, it is difficult to attain good reproducibility in regard to evenness of substrate temperature and evenness of feedstock gas flow. PA0 (1) Since DC magnetron sputtering is an established technique for forming a large-area thin film, it is advantageous in producing a large-area thin-film solar cell. PA0 (2) High-speed formation of a transparent conductive film is possible, and a high production efficiency is attainable in industrial production. PA0 (3) Resistivity can be regulated by regulating the content of an alloying element, e.g., aluminum, in a ZnO target, so that it is possible to form a transparent conductive ZnO film having a thickness reduced to 1/2 to 2/3 of those of the transparent conductive ZnO films formed by MOCVD. PA0 (4) Since a transparent conductive film can be formed in a short time period, the substrate is less apt to suffer deformation by thermal stress, and the film peeling caused by substrate deformation is less apt to occur. Further, the substrate need not be heated. Accordingly, it is possible to deposit transparent conductive ZnO films without heating a substrate. PA0 (5) A homogeneous film can be deposited from the initial stage of the formation of a transparent conductive ZnO film. PA0 (6) It is possible to deposit a transparent conductive ZnO film having a regulated resistivity value by utilizing a reactive sputtering method using a target of zinc metal or a target of an alloy of zinc with aluminum or any of other various dopants (boron, indium, etc.) and using an oxygen mixture gas. PA0 (1) Formation of a transparent conductive ZnO film comparable in performance to those formed by MOCVD is difficult with any known DC magnetron sputtering technique. PA0 (2) DC magnetron sputtering uses a higher energy density than RF sputtering, so that the junction interface or the light absorbing layer surface is apt to be damaged. PA0 (3) Since the concentration of an alloying element for resistance regulation in the target is constant throughout the target, resistivity cannot be freely regulated. (However, resistivity regulation in a narrow range is possible by incorporating oxygen into the sputtering gas.) In order to reduce the resistivity increasing the concentration of an alloying element in the target, the transparency of a ZnO film is significantly degraded and functionally insufficient as a window layer of a CIGS thin-film solar cell. PA0 (4) It is impossible to form a transparent conductive film having a surface texture with controlled roughness when the sputtering is conducted at a substrate temperature within the range in which a high-performance CIGS thin-film solar cell can be produced.
Processes for producing zinc oxide (ZnO) for use as such transparent conductive film include the metal organic chemical vapor deposition method (MOCVD) and sputtering. Features of each method are shown below.
The advantages of MOCVD include the following.
The disadvantages of MOCVD include the following.
Conventional methods of sputtering for forming a transparent conductive film include RF sputtering and DC sputtering. Specifically, a transparent conductive ZnO film has been formed by RF or DC sputtering on a heated substrate in one step using an argon gas or O.sub.2 /Ar mixed gas as a sputtering gas and using a ZnO--Al target or the like as the only target.
Few examples have been reported on the fabrication of a CIGS thin-film solar cell using DC sputtering. All the high-performance window layers-obtained by sputtering have been films deposited by RF sputtering using a target of ZnO--Al or the like. However, the rate of film deposition by RF sputtering is low and is almost the same as that by MOCVD.
Features of DC magnetron sputtering are shown below.
The advantages thereof include the following.
The disadvantages of DC magnetron sputtering include the following.
In FIGS. 3(a) and (b) are shown the results of a comparison in cell properties between solar cells employing a transparent conductive ZnO film formed by DC magnetron sputtering (indicated by the symbol ) and solar cells employing a transparent conductive ZnO film formed by MOCVD (indicated by the symbol .oval-solid.).
Whether a transparent conductive ZnO film is satisfactory or not can be judged by evaluating, among various solar cell properties, the series resistance of the film itself based on short-circuit current density (J.sub.sc) and by evaluating the influence of the light absorbing layer surface or junction interface on the junction characteristics of the interface based on the fill factor (FF), which is a measure of interfacial junction characteristics in solar cells.
From the results given in FIGS. 3(a) and (b), it was found that the solar cells employing a transparent conductive ZnO film formed by DC magnetron sputtering had lower values of both short-circuit current density (J.sub.sc) and fill factor (FF). It should be noted that the above transparent conductive ZnO film is one formed by DC magnetron sputtering using a single ZnO-Al target and is a film for use as a single-layer window layer.
The above results show-that since the single-layer transparent conductive ZnO film formed by DC magnetron sputtering has a high content of components which contribute to the series resistance of the film, the short-circuit current density (Jhd sc) and fill factor (FF) and low. Due to the plasma damage by DC magnetron sputtering, the junction characteristics are impaired. Namely, since DC magnetron sputtering uses a high energy density, it has drawbacks that the transparent conductive film formed has a high content of components contributing to recombination and that there is a fear of causing damage to the junction interface or to the light absorbing layer surface, although the sputtering technique is capable of high-speed film formation.